Dipole
subwoofers are quite different to the other subwoofer systems described on this site
because of the way they treat the the driver's output. Your typical subwoofer driver
produces sound from both the front and the rear of the cone, and the output from the
rear is out of phase with the output from the front, which results in very reduced
response levels, unless the rear wave is treated in some fashion. The other
subwoofer systems described on this site all employ some means of dealing with
the driver's rear radiation to improve overall low frequency response, the result being a
"monopole" bass system that theoretically has the same response
characteristics in all directions. However, for dipole bass systems, the rear radiation is
left untreated, and instead the overall response of the system is adjusted by varying the
size of the baffle and the "Q" of the system to achieve the best overall
response characteristics.

Driver CharacteristicsThe drivers used in dipole systems tend to be quite different to those in
"monopole" bass systems. The driver's Qts tends to be particularly high
(in some cases, as high as 2.0), the idea being to introduce a "bump" in the
driver's frequency response around Fs that will compensate for the 6dB/oct rolloff in the
response that will occur when the driver is mounted in an open baffle.
Alternatively, a "normal" driver can be used in a dipole bass system, but a
considerable amount of equalization may have to be used to make up for the loss in low
frequency performance.

Response CharacteristicsA dipole bass system has a "figure of eight" response pattern, which is
entirely different to the "spherical" response pattern of your typical
"monopole" subwoofer. The system's output is most powerful directly in
front and behind the baffle, and decreases to zero at the sides, where the front and rear
waveforms cancel each other. This response characteristic is said to be one of the major
advantages of a dipole bass system, as the restricted dispersion results in fewer boundary
reflections, which in turn is supposed to result in a smoother in-room response.

DrawbacksDipole bass systems tend to be rather large, employing multiple drivers,
primarily to make up for the output reduction due to the 6dB/oct baffle loss. This is not
the type of system to use if you've got a small living room, and it's certainly not
suitable for car audio!

Baffle SizeThe response of the system will be affected by a 6dB/oct drop in output below a
particular frequency referred to as Fequal, that's directly dependent on the size of
the baffle. At Fequal, the magnitude response (SPL) of the baffled driver will be
the same as its infinite baffle response. Above Fequal, the response will rise to a 6dB
peak at Fpeak (approximately equal to 3*Fequal), and at higher frequencies, the
response will depend largely on the shape of the baffle. A completely circular
baffle will produce the worse response characteristics, with deep nulls at multiples of
Fpeak.

The following table demonstrates the relation between the baffle's
effective diameter (i.e. the diameter of a circular baffle that has the same radius as the
smallest dimension of the baffle), Fpeak, and Fequal:

Speed of sound, c=344 m/s

Diameter (metres)

Fp (Hz)

Fe (Hz)

0.43

800

267

0.57

600

200

1.15

300

100

1.43

240

80

1.56

220

73

1.72

200

67

From the table, it's plain to see that it's nearly impossible to push Fequal
much lower than 80 Hz unless a fairly large baffle is used. The tradeoff here is
efficiency; the smaller the baffle, the lower the final efficiency of the dipole system.
OTOH, the larger the baffle, the higher the efficiency, but response at the upper
end of the passband could get somewhat irregular as Fpeak is reduced.

Almost all dipole bass designs incorporate some means of boosting the
response at low frequencies to compensate for the baffle loss. Typically one or more
of the following methods are used:

A high-Q driver is employed (the high Q results in a peak in the driver's
free-air response at its resonance frequency).

The Q of the system is increased by employing a series resistor (Qes is
increased, which results in an increase in Qts).

Active equalization is used to boost the low frequency response.

Active or line-level filtering is used to cut the higher frequency levels
to match the low frequency response.